1. Field of the Invention
This invention relates generally to target detecting devices for missiles and particularly relates to an active optical target detector including means for minimizing the adverse effects of backscatter of optical radiation.
2. Description of the Prior Art
It is well known that every missile is provided with a target detecting device. Such a device will detect a target and identify the closest approach by the missile to the target. When this occurs, the detector will generate a signal which causes the warhead of the missile to detonate. If at that time the missile is close enough to the target, the target will be destroyed.
A particular form of target detector is an active optical target detecting device. It consists of one or more sets of transmitters and receivers operating in the optical spectrum, which may be defined as radiation having a wavelength between approximately 0.2 micron and approximately 15 microns, one micron being one millionth of a meter.
Optical radiation in this range is subject to scattering by small particles or droplets found in the atmosphere. Such water droplets may be formed by rain or fog. These particles will scatter or reflect optical radiation depending on the particle size and the wavelength of the radiation. A receiver or receivers associated with the transmitter will pick up the so-called backscatter which represents noise. This noise in turn may swamp the desired signal which is the radiation reflected by a target.
A method has been proposed in the patent to Goggins, Jr., 4,005,414, for reducing the effects of radar backscatter. It has been found that the backscatter is a function of the polarization of the radar signals. Hence, it is proposed to transmit circularly polarized signals and to receive return signals which are linearly polarized. This in turn makes it possible to determine the major axis of the target.
The patent to Lehmann, U.S. Pat. No. 3,413,633, describes a method for searching for targets in space; that is, for detecting celestial bodies. To this end electromagnetic waves at a frequency near 100 mHz are transmitted. These waves are swept across a predetermined portion of space.
The patent to Rosen et al., U.S. Pat. No. 3,531,803, discloses apparatus for despinning an antenna beam from a spinning body. The antenna includes an array of waveguides carried by the spinning spacecraft. Hence, the radiated frequency is in the microwave region. In order to eliminate the effects of the spin, the waveguides are sequentially energized so as to direct a beam in a predetermined direction. The patent only discloses a transmitter and not a receiver.
The patent to Holmes et al., U.S. Pat. No. 3,913,485, is directed to an active/passive fuzing system. The energy transmitted is in the radio frequency energy region. A single transmitter and two receivers are simultaneously operated to provide a fuzing system which is effective in spite of any countermeasures and other interfering signals.